1. Field of the Invention
The present invention relates to a power switching device used for controlling a motor or the like, and more particularly to a semiconductor device including a switching device and an overvoltage protective circuit.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a semiconductor device including a conventional overvoltage protective circuit. The semiconductor device employs an n-channel MOSFET 1 as a switching device connected between load terminals 5 and 6. An overvoltage protective circuit 40 is provided for protecting the MOSFET 1 from an overvoltage. The overvoltage protective circuit 40 comprises an avalanche diode 2 that generates an avalanche current when an overvoltage is applied, and a current limiting resistor-diode 3 for restricting the avalanche current. The diode and resistor are connected in series between the drain electrode D and the gate electrode G of the MOSFET 1. In addition, a reference (Zener) diode 4 for protecting the gate electrode G from the overvoltage is connected between the gate electrode G and the source electrode S. Furthermore, the gate voltage is applied from a control input terminal 9 to the gate electrode G through a gate resistor 8. Thus, the MOSFET 1 is controlled to enter the OFF state, that is, the voltage blocking state by applying a low gate voltage.
Unless the overvoltage protective circuit 40 for protecting the MOSFET 1 from an overvoltage is provided an overvoltage that exceeds the withstanding voltage of the MOSFET 1 applied thereto will cause the avalanche current to flow through the MOSFET 1, resulting in damage of the MOSFET 1 when the avalanche current is excessively large. Generally, the avalanche current intensely flows through locations where the electric field is concentrated such as locations where the curvature of the PN junction is small, which PN junction is formed to establish the withstanding voltage. Accordingly, the current density is liable to increase in such locations. This may often cause damage of the MOSFET 1 even if the avalanche current is rather small per se.
On the other hand, the overvoltage protective circuit 40 as shown in FIG. 1 can protect the MOSFET 1 from such damage by setting the avalanche voltage of the avalanche diode 2 below the withstanding voltage of the MOSFET 1. More specifically, by setting the withstanding voltage of the avalanche diode 2 below that of the MOSFET 1, the avalanche current flows first through the avalanche diode 2 rather than the MOSFET 1 when overvoltage occurs across the load terminals 5 and 6. Since the avalanche current flows through the gate resistor 8 to the control input terminal 9, a voltage drop takes place across the gate resistor 8. Thus, the gate voltage increases and the MOSFET 1 conducts. As a result, the energy of the overvoltage across the load terminals 5 and 6 is absorbed in the form of the ON current of the MOSFET 1. This reduces the voltage across the load terminals 5 and 6, which in turn drops the avalanche current flowing through the avalanche diode 2. Thus, the MOSFET 1 tries to change into the OFF state again. In practice, however, the avalanche current of the avalanche diode 2 balances the current flowing through the MOSFET 1 so that a constant current flows through the MOSFET 1, which current is determined by factors such as the impedance of the voltage source supplying the overcurrent. Thus, the overvoltage protective circuit 40 protects the MOSFET 1 from the overvoltage by preventing the avalanche current from flowing through the MOSFET 1. Further, as long as the impedance of the voltage source is not too small, the avalanche current flowing through the avalanche diode 2 does not increase to a degree that damages the diode 2, thereby protecting the semiconductor device in its entirety.
Thus, the semiconductor device comprising such an overvoltage protective circuit can protect the switching device from damage by setting the avalanche voltage of the avalanche diode 2 below the withstanding voltage of the switching device. However, considering variations in the avalanche voltage of the avalanche diode, the withstanding voltage of the switching device must be set sufficiently higher than the avalanche voltage. The high withstanding voltage of the switching device, however, increases the ON state voltage and the switching loss of the switching device because the high withstanding voltage requires a thick drift layer through which the depletion layer spreads.
Furthermore, with regard to bipolar devices such as an IGBT (Insulated Gate Bipolar Transistor), when an inductive load such as a motor is turned off, an overvoltage takes place across the emitter and collector because of the -di/dt due to the inductive component such as stray reactance. As a result, although the gate voltage begins to drop and the current begins to reduce in this case, the base voltage of the NPN transistor constituting the IGBT increases. This causes a constant drain current to continue to flow. Thus, the minority carrier moves through the depletion layer spreading through the PN junction, and the electric charges of the minority carrier strengthen the electric field in the depletion layer. Accordingly, it is necessary to consider the reduction in the withstanding voltage from the static withstanding voltage in the IGBT or the like in the case of a turnoff operation. This will require a further increase in the margin between the withstanding voltage of the switching device and the avalanche voltage. As a result, the device characteristics such as an ON state voltage of the bipolar devices like an IGBT tend to be further degraded.